Method for treating a hot-rolled metal body and the like

ABSTRACT

A hot-rolled metal body emerges from between the last two rolls of the rolling-mill train in a hot, solid, and relatively scalefree condition and passes immediately into a treatment chamber prior to any substantial formation of scales on the body. In this chamber a powder, consisting of a mixture of reducing agents and substances which fuse at the temperature of the body, are blown against it and melt. On cooling, this melt hardens to form a glassy, enamellike coating which breaks in the region of any surface flaws to indicate the presence of the latter and protects the body from further scaling as it further cools.

United States Patent Schaumburg Mar. M, 1972 [54] METHOD FOR TREATING AHOT- 2,458,715 1/1949 Marencik et a1 .29/424 ROLLED METAL BODY AND THELIKE 2,762,115 9/1956 Gates ..29/424 3,169,310 2 1965 G'b t l ..29 [72]Inventor: Georges Schaumburg, bis rue de Londres, 1 son 6 a /527 4Momlgny'les'MetZ France Primary ExaminerJohn F. Campbell [22] Filed:June 23, 1969 Assistant Examiner-Victor A. Di Palma [21] PP 835,359Attorney-Karl F. Ross [57] ABSTRACT 30 F A l' P rlt 1 reign pp cation noy Data A hotrolled metal body emerges from between the last two June 24,1968 Germany ..P 17 71 666.4 rolls of the rolling-mill train in a hot,solid, and relatively scale-free condition and passes immediately into atreatment 2 /4 9 4 9. ,2 .-.9; chamber prior to any substantialformation of scales on the 7/23 body. In this chamber a powder,consisting of a mixture of [51] Int. Cl ..B23q 17/00 reducing agents andSubstances which fuse at the tempetawre [58] held of Search 2 1 of thebody, are blown against it and melt. On cooling, this I l3 117/2 8/23melt hardens to form a glassy, enamellike coating which breaks in theregion of any surface flaws to indicate the [56] References cuedpresence of the latter and protects the body from further scal- UNITEDSTATES PATENTS mg as It further @015- 2,442,485 6/1948 Cook ..117/50 2(Ilaims, 7 Drawing Figures c mnmm com-inn PATENTEDMAR 14 I972 3, 648 349 sum 1 ur a INVENTOR. GEOB GES SC'HM/HBUEG ATTORNEY PATENTEDHAR 14 m2SHEEY 2 0F 3 m tlllllrllbiL lxiifll r wuFumFoum mJmmXmJ E h w uINVI'JN'I'OR. GEORGES SCHA PATENTEDHAR 14 m2 3. 648 349 sum 3 or 4INVLNTOR.

GEORGES SC'HAUMBUQG BY ss K rl Tn ATTORNEY METHOD FOR TREATING AHOT-ROLLED METAL BODY AND THE LIKE FIELD OF THE INVENTION The presentinvention relates to a method of and an apparatus for treating ahot-formed metal body and, more particularly, a hot-rolled metal body.

The invention also relates to a method of and an apparatus forhot-rolling of steel bodies and to a technique for detecting surfaceflaws or defects thereon.

BACKGROUND OF THE INVENTION In the rolling of a metal body, a heatedingot, billet, bloom or slab is passed between a plurality of pairs ofrolls of successively narrowing spacing so that the slab or ingot islengthened and narrowed. This operation is preferably carried out withthe billet in a hot condition.

As the body leaves the last pair of rolls in the rolling train it isquite hot and relatively scale free, since the mechanical compression ofthe body effectively descales it. This condition does not last longsince the body starts immediately to scale at high temperatures in theambient oxidizing atmosphere. The scaling is detrimental in itself, inthat it roughens the exterior of the workpiece, and has the furthereffect of so obscuring or hiding any surface flaws on the workpiece thatthey pass notice altogether, thereby giving rise to an imperfectfinished product. Removal of these scales is often carried out by atime-consuming operation, such a pickling or sandblasting, so thatproduction is also hindered by the need for these steps.

OBJECTS OF THE INVENTION It is, therefore, the general object of thepresent invention to provide an improved method of and an apparatus fortreating a hot-formed metal body.

A more specific object is to provide such a method and apparatus whichovercome the above-mentioned difficulties particular to hot-rolled metalbodies.

It is another object of this invention to provide an improved method ofdetecting surface flaws in hot-formed bodies and especially hot-rolledsteel bodies.

Still another object of this invention is to provide a method forprotecting hot-rolled steel bodies against scaling and also to improvemarkedly the quality of hot-rolled steel bodies obtainable from arolling mill.

SUMMARY OF THE INVENTION The above objects are attained, in accordancewith some features of the present invention, by a method of treating ametal body which emerges in a hot, solid, and relatively scalefreecondition from a hot-forming stage, specifically in a rolling mill, andpasses directly into a treatment chamber. A powder consisting of amixture of reducing agents to remove any slight scales and glass-formingsubstances which fuse at the temperature of the body, is blown againstthe exterior of the body as it passes through this chamber. The powderfuses and, on cooling of the body, solidifies to form a glassy,enamellike coating which ruptures in the region of surface flaws on thebody, thereby enabling their observation, while sealing the body offfrom the atmosphere to prevent any further scaling.

In accordance with another aspect of the invention, the method involvesthe steps of hot-forming a steel body and, prior to any substantialformation of scale thereon or after an initial treatment of the hotslightly scaled body with a reducing agent, forming a thin flowableliquid-glass or enamel coating on the body. Upon hardening of thiscoating, it not only seals the surface against chemical attack andscaling, but (of even greater importance from the point of view of thisaspect of the invention) itself is indicative of the presence of surfaceflaws as noted below. These flaws, not readily visible otherwise, canthen be eliminated by conventional techniques.

This treatment chamber, according to further specific features of thepresent invention, is fitted with at least one nozzle which is connectedthrough at least one blower to a hopper containing the above-describedpowder. The nozzle is formed with a Venturi restriction and a small pipeopening longitudinally into the nozzle is connected to a source of gasunder pressure to accelerate the powder and project it out of the nozzleagainst the body. An inert gas, a reducing gas, or simply air can beused as propellant.

The powder can be a mixture of oxides of silicon, magnesium, aluminum,and iron, along with metallic sodium, boron, or potassium, or a compoundof these chemicals. Zinc borate, according to a further feature of thepresent invention, has proven useful in the enamel layer in that thefinal product is effectively zinc coated upon removal of the coating.

In accordance with yet a further feature of the invention, the coatedand cooled body can be stripped of its temporary glassy coating, forinstance in a straightening station, and subsequently recoated forgreater permanence by spraying with paint, lacquer, a synthetic-resinprotective coating, or the like. Such a method is particularlyadvantageous in that the glassy coating is quite easy to remove from thebody, i.e., more easily removed than the customary scale, so that thebody can be completely processed in one continuous operation.

Prior to the enamel coating, it is possible to subject the body totreatment with a reducing agent, such as hydrogen or methane, to removeany slight scale thereon. In this case, there is no need to admixreducing agents such as powdered reducing metals, with the powder.

In an alternative embodiment the body, which can be a continuous rod,wire, sheet or the like, is simply passed through a fluidized bed of theabove-described metal and metal-oxide powders to coat it. In any case,the coating must be thin enough to rupture in the region of any flaws,or to allow the flaws to be seen through the coating, while being toughenough not to flake off immediately as the workpiece is transported.

Thus, a key feature of the present invention is the formation of aglasslike coating upon the surface of a hot-formed body while it isstill in the heated condition and susceptible to sealing, of arelatively thin and brittle character upon cooling. This enamel layer,which may be composed of any conventional enameling composition,dispensed in the liquid or solid state and merely solidifiable on thebody and/or fusible thereon utilizing the heat of the body, surprisinglyis capable of providing a visible indication of surface flaws or defectsin the body. While the mechanism of this system is not fully understood,it would appear that the enamel film ruptures, fractures or breaks inthe presence of surface flaws, e.g., pits, striations, projections,contamination areas, cracking and similar defects, either as aconsequence of thermal stress during cooling, or mechanical movement ofvarious portions of the defect area, etc. or as a result of an inabilityto adhere effectively to these regions, thereby producing a fracturezone in the region of the defects. This marking or indication of thedefects permits the defective material to be removed or otherwisemonitored to ensure that the finished product will be free from theflaws. The enamel layer does not remain as tenaciously upon the body aswould a slag layer or other scale-forming material, but rather can bebroken away with ease as will be apparent hereinafter.

While reference has been made above to enamel and glasslike material ascapable of fulfilling the requirements of the present invention, it maybe noted that numerous enamel compositions which are capable of meltingat a wide range of temperatures including the temperatures at which themetal (steel) body emerges from the hot-forming stage, are known and maybe used in accordance with the present invention as long as the materialis applied to the body with a viscosity sufficiently low to enable theliquid material to flow into a uniform thin film coating the body. Thisfilm, which may have a thickness of the order of microns, e.g., 5-10microns, may either be applied in the form of a liquid coating or glassor may be applied in a powder stream, e.g., designed to fuse to flowalong the surface at the temperature of the body, but in either casewill be a glassy substance.

When powders are used, it is preferred to direct them against the bodyin high-velocity aerosol-type jets, the velocity being controlled bycompressed air. At sufficiently high velocity, the powder effectivelypenetrates the surface skin of the metal, which can be assumed to besubstantially free from scale, and is rapidly heated by its intimatecontact with the metal to the melting point of the powder andthereabove, whereby the molten droplets coalesce to form a liquid glassfilm upon the surface of the metal. Preferably, this treatment iscarried out in a tubular chamber enclosing the body and open at itsends-to permit the body to pass through the chamber. The latter isprovided with a configuration geometrically similar to that of the body,i.e., a chamber of circular cross section when the body is acontinuously cast rod of circular cross section.

As noted earlier, an important feature of this invention resides in theprovision of reducing agents in the treatment process to eliminate anyscale which may have been formed on the body prior to coating with theenamel layer. Thus, the body may be treated with a reducing gas orreducing agents, e.g., sodium, potassium and like reducing metals may beemployed, the reduction products and the oxidized metal compoundsresulting from the reducing reaction being incorporated in thesubsequently formed enamel coating. In this respect, it may be notedthat scale layers, when composed of the slag-formed present inmetallurgical systems are of a glassy nature and are soluble in theglasslike enamel coating applied in accordance with the presentinvention.

In applying the powders, preferably in the form of a powder mixture ofthe character set forth above, it has been found to be advantageous toprovide a tandem array of blowers, the first receiving a mixture ofpowders and air and constituting a loosening means whereby the powdersare dispersed in the airstream to form a flowable mixture. In the secondor downstream blower, the powder/air mixture is accelerated to a highvelocity and propelled to a discharge nozzle trained upon the hot-metalsurface. Between this nozzle and the second blower, there may beprovided a venturi injector to which compressed air is fed to regulatethe velocity of the powder/air mixture directed against the workpiecesurface.

While the invention is effective with hot-formed metals of any type, ithas been found that hot-rolled products deriving from a rolling mill inwhich hot ingots or blooms are rolled in a heated state, and continuouscasting arrangements offer the best sources of the bodies which aretreated in accordance with this invention. Of course, the stepsenumerated above apply whether the bodies are rolled or continuouslycast, and either type of body may be subjected to the subsequenttreatment as indicated earlier to form a final and more or lesspermanent coating therein.

DESCRIPTION OF THE DRAWING The above and other objects, features, andadvantages will become more apparent from the following description,reference being made to the accompanying drawing, in which:

FIG. I is a perspective view of a rolling mill embodying the principlesof the present invention;

FIG. 2 is a vertical section through the rolling mill of FIG. 1;

FIG. 3 is a perspective view of a continuous casting installationembodying the principles of the present invention;

FIG. 4 is a vertical section through the installation of FIG. 3;

FIG. 5 is a vertical section through another apparatus embodying thepresent invention; and

FIGS. 6 and 7 are sections through a metal body at two separate stagesin its treatment according to the present invention.

SPECIFIC DESCRIPTION As seen in FIGS. 1 and 2, a slab or sheet 3 isformed between two rolls 23 and 23' of a last rolling stage 1 of arolling train. This slab 3 emerges from between these rolls 23 and 23'in a very hot, solid and relatively scale-free condition. It istransported by driven rollers 2 through a chamber 22 where it issubjected to the reduction effects of a reducing gas, e.g., hydrogen,carbon monoxide, methane or mixtures thereof, to remove any slight scaleformations which might have come into being thereon.

The slab 3, still hot, then enters an inlet 4a of a treatment chamber 4.As it passes through this chamber 4 it is contacted on all its surfacesby a powder P which is a mixture of reducing-metal and metal oxideparticles that fuse on the surface and form a very hard, inflexibleenamellike glassy coating (see FIG. 6). In addition, other substances(e.g., zinc) needed for the later surface treatment of this body can beadded to this powder in the form of the material itself or as anothersubstance (e.g., zinc borate) which produces the desired coating oralloying substance in situ.

These powders are held in a hopper 11 which is closed by a vane 17 andconnected through a conduit 12 to a funnel 26 feeding a first aeratingblower 9 in turn connected to a second driving blower 10. A conduit 13leading from this second blower 10 is formed with a Venturi restriction15 just downstream from the end of a narrow pipe 14 into whichcompressed air is fed. Thence the aerosol particles P are expelledthrough a nozzle 8 onto the slab 3 all around the latter. Thus anothernozzle 18 can be connected also to the blower 10 and the pipe 14 tocause the fine particles P to impinge against the underside of the slab3 in a region 27 free of rollers 2.

After the treated slab 3 exits through an outlet opening 4b of thechamber 4 it passes along a region 5 where it cools and the coatingbecomes very hard. FIG. 6 shows the slab 3 with its hardened coatings C.

Further downstream the slab 3 passes through a straightening station 6where three rollers 28a c bend the slab 3 sufficiently to break free thecoating C. Due to the regularity of this coating, it is easily removed.This coating is, however, very tight on the metal body 3, since thesurface of the body 3 reacts to some extent with the wetted powders P toform the coating C.

Finally a coating station 7 having a plurality of nozzles 7a sprays amore flexible protective coating M, e.g., paints, upon the surface ofthe cooled slab 3. This coating M is shown in FIG. 7.

Because of the thinness of the coating C, any flaws, as shown at F inFIG. 6, will not be covered; the coating C will break in the region ofthese flaws F. Thus, an observer in the region 5 can clearly see themand mark the slab 3 in this region so that it can be later discarded.

In this manner, the flaws are easily recognized and there is no need todo a major descaling of the sheet 3, since any slight scale is removedby the gas in the chamber 22 or the oxides sprayed against it in thechamber 4 and, for the rest of the cooling time, the sheet 3 is coveredby a very tight, nearly continuous glassy coating. Removal of thiscoating C has been found to be very simple once the sheet 3 is fullycooled and further scaling is no longer a problem.

FIGS. 3 and 4 show an alternative embodiment of the present invention.Here, molten metal is poured from a ladle 16 into a funnel la on top ofa mole 1' of a continuous casting installation. A metal body in the formof a thick wire 3' leaves the base of this mold l and is bent through bydriven rollers 3 of hyperbolic profile and through a chamber 22' similarto the chamber 22 of FIGS. 1 and 2.

A boxlike chamber 4' is equipped with two nozzles 18 and 8' connectedthrough a powder-aerating apparatus to a hopper 11' just as in FIGS. 1and 2, common reference numerals referring to common structure. Thiswire 3 is here coated according to the method and principles set out inconnection with FIGS. 1 and 2.

As the wire 3' leaves the box 4', it is gripped between rollers 5'. 1

In this embodiment, the enamel coating is not stripped off the wire 3',since it serves to protect the wire during shipping and handling, andrelatively easily removed when desired. This coating also serves toaccentuate and make more visible flaws in the surface of the wire 3', asdescribed above for FIGS. l and 2.

FIG. 5 shows a further embodiment of the invention having in this case atreatment chamber 4". Here a fluidized bed is formed around a continuousworkpiece 3" reiding on rollers 2". The workpiece 3" is very hot andrelatively scale free. A blower 21 having an output connected through aconduit 20b to the bottom of the chamber 4" and an input conduit 20aconnected to the top of this chamber 4" serves to fluidize minutemetallic and metallic oxide particles P fed into the chamber through aconduit connected to a hopper 11" above the chamber 4". In this way theworkpiece 3 is very efficiently covered by the powder, so that it canmove at high speeds through the chamber 4" and still be adequatelycoated.

SPECIFIC EXAMPLE EXAMPLE A A steel body is hot-formed and enters thetreatment chamber at a temperature somewhat below its melting point. Itis sprayed with a mixture of powders such that substantially equal partsof SiO AI..O Fe o CaO, MgO, Na, K, and B mixed with and carried by airstrike the surface of the body. This mixture reduces the slight scale onthe surface and fuses to form a glassy enamellike coating over theentire exterior of the body, the fused mixture flowing to cover all ofits surface to a thickness on the order of say 5 to microns. As the bodycools, the enamel coating breaks in the region of any flaws on it.

EXAMPLE B A steel wire is formed by rolling and enters a treatmentchamber as above. In this chamber it is: sprayed with a powder whoseprime constituent is zinc borate. The hard, enamellilce coating isformed as above. Then the body is fed through a straightening apparatuswhich removes the coating and is found to be zinc coated in the sensethat zinc is alloyed with surface zones of the metal body.

Iclaim:

l. A method of accenting and removing surface flaws in a hot metal bodyemerging in a substantially scale-free state from a forming stage,comprising the steps of:

a. applying a glassy coating of a thickness of about 5 to 10 microns tothe surface of said body by continuously depositing on said surface ofsaid body as it emerges from said forming stage a glass-formingsubstance fusable at the temperature of said body and prior tosubstantial scale formation thereon;

b. selectively cracking said coating in the region of surface flaws ofsaid body by cooling said coating on said body; and

c. removing said flaws from said body at regions thereon visuallyaccented by cracks in said coating.

2. The method defined in claim 1 wherein said forming stage is a rollingstage from which said body emerges continuously and said substance is apowder containing a reducing agent and applied to said body by blowingsaid powder thereagainst.

2. The method defined in claim 1 wherein said forming stage is a rolling stage from which said body emerges continuously and said substance is a powder containing a reducing agent and applied to said body by blowing said powder thereagainst. 